The flower colors of orange, red, violet and blue primarily are provided by flavonoids referred to as anthocyanins. Although yellow is mainly provided by compounds other than flavonoids, such as carotenoids, betalains, etc., the yellow color of some plants is provided by flavonoids. For example, compounds classified as aurones are known to be present in the petals of some varieties of snapdragon, limonium, morning glory, dahlia, strawflower, Jerusalem artichoke and cosmos (Saito: BIO HORTI 1, 49-57, 1990).
Known examples of aurones include 4′,6-dihydroxyaurone, 4,4′,6-trihydroxyaurone, aureusidin, sulfretin and bracteatin, with aureusidin and bracteatin being contained in snapdragon, aureusidin contained in limonium, aureusidin contained in morning glory, sulfretin contained in dahlia, bracteatin contained in strawflower, and sulfretin contained in Jerusalem artichoke.
In addition, aurones are known to be contained in the plant of the family Compositae including the genera Coreopsis, Helianthus, Tithonia, Zinnia and Viguiera; the family Ericadeae including the genus Vaccinium; the family Cyperaceae including the genus Cyperus; the family Leguminosae including the genera, Acacia, Pterocarpus and Soja; and the family Rubiaceae including the genus Mussaenda (The Flavonoids, edited by J. B. Harbone, 1988, Chapman & Hall, 340-342).
The synthesis pathway of anthocyanins has been extensively researched, and with respect to the biosynthesis of aurones, it has been suggested based on its structure that 4′,6-dihydroxyaurone is synthesized from 2′,4,4′-trihydroxychalcone, and it has been proposed that peroxidase is involved in that reaction (Rathmel and Bendall, Biochem. J. 127, 125-132, 1972). However, there are no examples of definitively measuring the biosynthesis reaction of aurones using petal extracts and so forth of plants, and there are no reports that clarify the manner in which the reaction occurs in plant petals. In addition, there are also no reports of purifying enzymes involved in aurone synthesis.